1. Field of the Invention
The present invention relates to a method for producing a diesel fuel oil from waste edible oils in which a so-called vegetable diesel fuel oil, of which cetane index is 45 to 70, flash point is 80.degree. C. to 210.degree. C. and kinematic viscosity at 30.degree. C. is 2.0 mm.sup.2 /s to 10.0 mm.sup.2 /s, is obtained from waste edible oils such as rape seed oil, sesame oil, soybean oil, maize oil, sunflower oil, palm oil, palm kernel oil, coconut oil, corn oil, safflower oil. The invention also relates to an apparatus for producing a diesel fuel oil from waste edible oils employed for carrying out the producing method.
2. Prior Arts
At present, in case of Japan, edible oils such as soybean oil mounting to about 700,000 tons, coconut oil mounting to 80,000 tons, cotton seed oil mounting to 50,000 tons, rape seed oil mounting to 700,000 tons, are used every year, and 90% of those edible oils are dumped as refuse without collection. A part of the dumped waste edible oil is carried to a refuse disposal plant in the form of mixture with other wastes and incinerated together with combustible refuse, and other part of the dumped waste edible oil is buried together with incombustible refuse. In the meantime, the remaining 10% of the waste edible oil is recycled to be employed as a raw material of soap or the like. As for the method for recycling the waste edible oil, several soap production technologies have been established including batch system comprising salting out method, continuous system comprising continuous boiling method, continuous saponification method, etc., and these soap production technologies basically comprises the steps of saponification, salting out, cleaning, and finishing boil.
It has been heretofore known that a fatty acid alkyl ester is obtained by transesterification of monoglyceride, diglyceride and triglyceride with alkyl alcohol (Organic Chemistry Handbook, issued by Gihodo, 1988, page 1407 to 1409). As for a technology for producing a diesel fuel oil from vegetable oil or waste edible oil, several attempts have been proposed. For example, the Japanese Laid-Open Patent Publication (unexamined) Hei 7-197047 discloses a method in which, with respect to 100 parts of waste edible oil, after obtaining a reaction product by heating 15 parts to 25 parts of methyl alcohol and 1 part to 2 parts of caustic soda to a reaction temperature of 45.degree. C. to 65.degree. C., the steps of separation by stationary layer separation, cleaning by warm water, and dehydration and drying by drying agent are carried out, whereby a diesel oil is finally obtained.
Further, the Japanese Laid-Open Patent Publication (unexamined) Hei 7-310090 discloses a production method comprising: a first step of adding alkali to a raw material of fats and oils, in such a manner as adding 15% or more by weight of methyl alcohol to 100% by weight of fats and oils and adding 0.2% by weight to 1.5% by weight of alkali to 100% by weight of fats and oils, and stirring them at a temperature of 50.degree. C. to 64.degree. C.; a second step of removing a precipitation of a product obtained in the first step by stationary layer separation or by centrifugation; a third step of removing methanol by heating and evaporating a supernatant obtained in the second step to a temperature higher than a boiling point of methyl alcohol; a fourth step of separating and removing a water phase from two separated phases, after adding water and acid to a product obtained in the third step for neutralization while heating to 70.degree. C. to 90.degree. C. thereby separating into a water phase and an oil phase; a fifth step of separating and removing a water phase, after adding water to the oil phase for washing while heating to 70.degree. C. to 90.degree. C.; and a sixth step of obtaining a purified fatty acid methyl ester by filtration after adding and stirring a clay to the oil phase obtained in the fifth step while heating to 100.degree. C. to 140.degree. C.
It is, however, unreasonable from the viewpoint of resource saving that most of the waste edible oil is dumped without being utilized, with the exception that a small percentage of waste edible oil is recycled as a raw material of soap. Moreover, when the waste edible oil is incinerated together with combustible refuse, a large amount of suspended particulate matters such as SOx, NOx, CO.sub.2, CO turning to air pollutants are produced, and it is reported that deadly poisonous dioxin may be produced. On the other hand, when the waste edible oil is buried together with incombustible refuse, a soil pollution is brought about. A part of the waste edible oil dumped from restaurant, food plant, home, etc. is actually discharged into the river or lake through, which is one of the major causes of water pollution.
Recent years, under the background of increasing social concern to global environmental problems, new concept and varieties of political means on the protection of environment have been proposed. In the International Environment and Development Convention held on November, 1993, a declaration was adopted to establish a policy of getting rid of mass consumption society, in the context of reducing the environmental load due to social and economical activities as much as possible, and constructing a continuously developing society. To cope with this, on December, 1994, the Japanese government made a cabinet decision on a master plan of environmental administration, and following goals were established as specific activity program for the protection of environment to be achieved within five years: (1) reduction of waste amount by 25%. (2) reduction of combustible refuse amount occupying in the waste amount by 30%. (3) reduction of fuel amount used in energy supply facilities, etc. by 10%. etc. To achieve the foregoing goal values, recycling of plastics, container refuse, package refuse, etc. and life cycle assessment have been introduced and, at the same time, varieties of researches and developments are going on. For example, technologies including liquefaction of plastics, immobilization of CO.sub.2, recycling of lubricating oil for automobile, degassing of NOx from exhaust gas have been already developed. However, the fact is that most of the waste edible oil is discharged to the environment of earth without being utilized, with the exception that a small percentage of waste edible oil is recycled as a raw material of soap.
It is certain that several attempts for producing diesel fuel oil from waste edible oil have been heretofore proposed as disclosed in the mentioned Japanese Laid-Open Patent Publications Hei 7-197047 and Hei 7-310090, but following problems exist in those known production methods:
(1) Because the waste edible oil to be utilized as a raw material is not pretreated, activity of alkali serving as a catalyst for ester exchange reaction is reduced by moisture, free fatty acid, etc., whereby conversion rate of the waste edible oil is lowered. Because a large amount of water is left in fatty acid methyl ester to be a target product, and because the waste edible oil obtained after being subject to a high temperature heating hysteresis contains a odor material produced by thermal decomposition and oxidation of vegetable oil and the odor material remains in the target product, the target product is inferior in quality. Further, because the waste edible oil contains a large amount of foreign substances or impurities such as dust, lard, solid component, not only the target product is inferior in quality but also pipe lines and valves of the production system may be clogged by those foreign substances. Furthermore, when any moisture exists in the waste edible oil, in the following reaction formula (where: R, R1, R2 and R3 indicate alkyl group, and M indicates metal), a side reaction of producing a fatty acid soap (reaction of saponification) is easy to take place, and a part of the produced fatty acid soap remains in the fatty acid methyl ester to be the target product, which brings about a turbidity. Such a fatty acid soap is difficult to separate and, after all, the product is inferior in quality. ##STR1##
(2) In any production method, sodium hydroxide is employed as a catalyst. However, it is to be noted that activity of sodium hydroxide with respect to the ester exchange reaction is not sufficient, and in order to improve the conversion rate of the waste edible oil to 90%, it is necessary to add methyl alcohol (stoichiometrically required amount of methyl alcohol is about 3 mole with respect to 1 mole of vegetable oil) and catalyst excessively. As the excessively added alcohol and catalyst should be removed from the target product in the subsequent process, the production process becomes complicated. Moreover, the alcohol and catalyst removed from the target product cannot be recycled, which results in increase in cost. When utilizing sodium hydroxide as a catalyst, a side reaction of producing a fatty acid soap (reaction of saponification) is easy to take place as shown in the foregoing chemical formula. And a part of the produced fatty acid soap remains in the target product, which brings about a turbidity. Such a fatty acid soap is difficult to separate and, after all, the product is inferior in quality.
(3) In the prior arts, to refine fatty acid methyl ester produced by reaction of the waste edible oil with methyl alcohol, sodium hydroxide remaining in the fatty acid methyl ester is removed through cleaning by warm water or neutralization by acid. Hence not only the production process becomes complicated, but also the reaction of saponification is accelerated all the more under the existence of warm water and sodium hydroxide. Moreover, it is difficult to remove completely the water after cleaning.
(4) Treatment of glycerol or how to recycle glycerol which is a by-product of the ester exchange reaction between the waste edible oil and methyl alcohol is not established, and after all glycerol becomes a refuse resulting in pollution of environment.
(5) Concerning the equipment for mass production of diesel fuel oil from waste edible oil and the industrial production method thereof, any fruitful performance has not been reported yet, and any industrial production technology capable of obtaining a diesel fuel oil satisfying the current JIS standard about gas oil has not been established until today.
The conventionally established soap production technology from the waste edible oil comprises the steps of saponification, salting out, cleaning and finishing boil, and other than the production of soap, this soap production technology cannot be applied to the production of diesel fuel oil, for example. It is certain that the reaction of composing fatty acid alkyl ester from edible oil by alcolysis reaction has become possible in the laboratory basis, but any technology and equipment for industrially producing diesel fuel oil has not been developed yet.
The diesel fuel oil has been conventionally produced mainly from gas oil residue of petroleum or from a product obtained by catalytic cracking or thermal decomposition of residual oil through the treatment such as reforming, desulfurization, denitrification, when required. Accordingly, such a conventional production technology cannot be applied to the production of diesel fuel oil.